The Bridge of Tom and Ray

RAY: In Borneo, there is a bridge connecting two islands. It is constructed of bamboo, lashed together with hemp, and it's been used for hundreds of years. The bridge is four miles long and it has a weight limit of 20 tons.

One day, a truck full of sedated pigs pulls up to the bridge, ready to cross. The officials stop him and say, "We have to weigh you."

Once on the scale, the truck weighs *exactly* 20 tons. The bridge is emptied. The driver is allowed to drive across the bridge.

As he drives across, a sparrow begins to follow alongside. Just when he's beyond the halfway point, the sparrow lands on the truck.

What does the driver do to keep the truck and the sedated pigs from plunging into the abyss? (Note: jumping out of the cab and into the abyss is not an option.)